Wavelength dependent excited state dynamics observed in canonical pyrimidine nucleosides

Epidemiological evidence indicates that damage to DNA/RNA initialized by ultraviolet (UV) radiation is associated with skin cancer. Wavelength dependence of DNA photodamage was proposed as early as 1990s and demonstrated later on. Unraveling the photo-activated dynamics involved in related reactions is essential. However, studies aimed at uncovering the wavelength dependent excited state dynamics in canonical pyrimidine nucleosides have not received enough attention. In this work, excitation wavelength dependent excited state dynamics of 2′-deoxy-thymidine (dThd) and oxy-uridine (Urd) are investigated in acetonitrile solutions by femtosecond broadband transient absorption spectroscopy. Varying the excitation wavelength leads to a significant difference in the branching of the excited state population at the Franck-Condon (FC) region, resulting higher fluorescence quantum yield with 285 nm pump but higher triplet state quantum yield under 267 nm excitation. Based on our results, a vibronic coupling regulated excited state relaxation mechanism is proposed. This mechanism information is important for understanding the formation of harmful photoproducts for DNA/RNA with different wavelength UV excitations.